Method and device for measuring the thickness of a liquid layer

ABSTRACT

Determining the thickness of the liquid layer on a rough, poorly reflecting surface with a radiation source transmitting a beam to the surface with the liquid layer, a radiation detector that receives the beam reflected from the surface and liquid layer, and the thickness of the liquid layer on the surface being determined on the basis of a measurement of the degree of gloss and/or a diffused light measurement by the beam received.

FIELD OF THE INVENTION

[0001] This invention relates in general to measuring the thickness of aliquid layer on a rough poorly reflecting surface by measuring gloss.

BACKGROUND OF THE INVENTION

[0002] This description refers to liquid layers on surfaces, inparticular to an oil film lying on the surface of a drum that is used ina printer. In this case, a liquid, oil film or oil layer is applied to acylinder, particularly on an application roller or an application drumof a printer, whereby the application roller is used to fuse a toner ortoner material by pressure and heat on a stock, as described below. Forexample, with electrophotographic printers, a toner material is appliedto paper sheets and subsequently fused on the paper sheet. To this end,heated application rollers are often used, which effectively make thetoner material stick to the paper sheet by force fitting with highpressure and heat, gripping the paper sheet from above and below.

[0003] The problem with this conventional fusing method is that, inbetween the heated application roller and the stock, in particular thetoner material, adhesive forces are exerted, making it difficult toseparate the heated application roller from the stock. As a remedy, aliquid layer is applied to the heated application roller, which ensuresa detachment of the heated application roller from the stock once thetoner material has been fused. To prevent any disadvantages, care mustbe taken that the thickness of the liquid layer always lies within acertain range and that it does not assume any values that are too big ortoo small. If the values of the liquid layer thickness are too small,the above-mentioned disadvantages occur, and if values of the liquidlayer thickness are too large, the disadvantages consist of oily, tooglossy printing results, and, particularly with duplex printing, of thefouling of the printer by the liquid applied.

[0004] It is thus desirable to determine the liquid layer thickness onthe surface and, if necessary to correct it. U.S. Pat. No. 5,001,353describes a method and a device for measuring the thickness of layers onthe moved sheet. To this end, an ultraviolet beam is directed to thelayer and the fluorescence emitted by the sheet, is captured by acamera. By a mathematical relationship between the beam captured by thecamera and reflected by the sheet and the thickness of the layer on theplate, the thickness of the layer can be determined. In this case, thegeneration of an ultraviolet beam and the preparation of a camera withcorresponding additional characteristics are necessary. Both areexpensive. Furthermore, a smooth surface is required with thisrecommended solution of the state-of-the-art, in order to obtain theappropriate measuring results.

SUMMARY OF THE INVENTION

[0005] In view of the above, this invention is directed to determining areliable thickness of a liquid layer particularly on a rough and poorlyreflecting surface. In measuring the thickness of a liquid layer on asurface, particularly measuring the thickness of an oil layer on aprinter, a radiation source emits a beam to the surface with the liquidlayer, a radiation detector receives the beam reflected by the surfaceand the liquid layer, and the thickness of the liquid layer isdetermined on the basis of a measurement of the degree of gloss by thereceived beam. Furthermore, a method is available for measuring thethickness of a liquid layer on a surface, in particular for measuringthe thickness of an oil layer on a printer, whereby a radiation sourceemits a beam to the surface with the liquid layer, a radiation detectorthat receives the beam reflected by the surface and by the liquid layer,and the thickness of the liquid layer on the surface is determined onthe basis of a measurement of the diffused light by the beam received.

[0006] The beam from the radiation source is preferably polarizedperpendicular to the plane of incidence of the surface and thepolarization direction of the beam from the radiation source and ananalyzer in front of the radiation detector is the same. In this manner,the beam received by the radiation detector is maximally filtered andduring the measurement of the degree of gloss, undesired diffused lightfrom the beam received by the radiation detector is also maximallyfiltered. As indicated by the measurements, a suppression of thediffused light cuts it approximately in half. The gloss desired duringthe measurement of the degree of gloss, which is reflected by thesurface and the liquid layer, can be obtained by polarization, and theanalyzer, that has the same polarization direction as the beam, canconsequently pass through and be received by the radiation detector.

[0007] With a special embodiment of the invention, the rotation angle ofthe cylinder is measured at certain distances and the thickness of theliquid layer of the cylinder to the respective rotation angle isallocated. This makes it possible to distinguish between the variousareas on the surface with respect to the thickness of the liquid layeron the surface. This characteristic is particularly useful, since thedistribution of the fluid on the surface of printers is uneven.Selection of the angle of incidence of the beam from the radiationsource with the Brewster angle achieves the best results with respect tothe measurement of the degree of gloss.

[0008] The invention, and its objects and advantages, will become moreapparent in the detailed description of the preferred embodimentpresented below.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] In the detailed description of the preferred embodiment of theinvention presented below, reference is made to the accompanyingdrawing, in which:

[0010]FIG. 1 shows a schematic block diagram of an embodiment of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

[0011] Referring now to the accompanying drawing, a schematic blockdiagram of an embodiment of the invention shown is a lateral view of adrum 10, which rotates in the direction, illustrated by the directionarrows, in particular an application roller of a printer. The drum 10 inthe printer is used to fuse toner applied to the stock to mesh it withthe stock. To this end, the drum 10 exerts pressure on the stock withtoner and it is also heated, such as by an internal heater, whereby theheat of the drum 10 melts a solid toner and contributes considerably tothe fusing of the toner on the stock. On the surface of the drum 10,there is a liquid layer 12, which in the following special case is anoil layer, which is applied to the drum 10 from a dispensing device 7via a metering roller 5 and a donor roller 6. The dose of liquid on thestock can be determined by the knowledge of the thickness of the liquidlayer 12 on the drum 10 and the knowledge of the material used to makethe drum 10 and of the stock.

[0012] The liquid layer 12 on the drum 10 is prepared in the customarymanner by the dispensing device 7 via a donor roller 6 and a meteringroller 5, which are incorporated in the dispensing device. The oil to beapplied on the drum 10 is located in a container of the dispensingdevice 7, which is collected by the surface of the metering roller 5made of metal, moving in the direction of the arrow, and by touching therubber-coated donor roller 6, it is rolled onto the latter. The meteringroller 5, which rotates in an oil bath of the dispensing device 7, on anon-woven material, moves in the direction of the arrow opposite thedonor roller 6 and transfer the liquid layer 12 to the drum 10 bycontact. For clarity purposes, the liquid layer 12 in FIG. 1 is drawnconsiderably thicker in comparison with the drum 10. The thickness ofthe liquid layer 12 is indicated in the figure with the dimensions c.The metering roller 5 and the donor roller 6 are driven by drum 10,which is driven by its own drive or by friction.

[0013] Another possibility of applying the liquid layer 12 to the drum10 consists of passing an oil-soaked cloth over drum 10. In the presentembodiment, the thickness of the oil layer of drum 10, in this case, anapplication drum or an application roller of the printer, is determined.It is also conceivable that the metering roller 5 and/or on the donorroller 6 can be used when determining the thickness of the oil layer.

[0014] The drum 10 has a rough surface, whereby the roughness of thesurface lies in the micrometer range. When oil is applied, it isdistributed first into the deep areas or valleys of the surface of thedrum 10, and which fills it up, until ideally a closed liquid layer 12is formed. Once the liquid layer 12 has been applied, an increase in thegloss of the surface is visibly observed. Gloss is a sense impressionthat occurs during the reflection of light on a surface and which isperceived by human eyes. A smooth, high-finished surface reflects thedirected light and without diffusion, whereby the angle of incidence andthe angle of reflection on the surface are the same.

[0015] The gloss of the surface of the drum 10 is a measurement forliquid layer 12 borne by the drum 10. A characteristic curve of thegloss as a function of the thickness of the liquid layer 12 can thus begenerated. In order to determine the thickness of the liquid layer 12, aradiation source 1 and a radiation detector 2 are arranged with the drum10. The radiation source 1 and the radiation detector 2 are arranged ina single housing in this example. The radiation source 1 emits a lightbeam with a certain polarization direction and with a flat angle in thedirection of the rotating drum 10 with the liquid layer 12. Theradiation source 1 can contain a laser device, which emits parallellaser beams; the radiation detector 2 can contain a photo diode arrayfor receiving the beam reflected by the surface. Various angles ofincidence of the light beam can be positioned for various degrees ofgloss.

[0016] The light beam from the radiation source 1 is schematicallyillustrated in the figure with an arrow from the radiation source 1 tothe drum 10, while the light beam reflected is schematically illustratedin the figure with an arrow from the drum 10 to the radiation detector2. The light beam is reflected by the surface of the drum 10 and theliquid layer 12. By selecting a flat angle that hits the light beam onthe surface of the drum 10, the portion of the gloss from the light beamis basically reflected by the surface of the drum 10 with liquid layer12 in the direction of the radiation detector 2; however, the undesiredspurious radiation is not directed and basically does not reach theradiation detector 2. The preferred angle of incidence of the beam onthe drum 10 is chosen in consideration of the surface condition of thedrum 10, i.e., the surface roughness, the preferential direction, thecurvature of the drum 10 and the reflectivity. Generally speaking, thegloss clearly increases if the drum 10 has a low reflectivity.

[0017] In front of the radiation detector 2, an analyzer 9 is arrangedthat filters the light beam reflected by the drum 10. The analyzer 9 hasa polarization direction that is identical to the polarization directionof the light beam from the radiation source 1. This means that onlylight from the polarization direction used is allowed to pass. Theradiation source 1 and the radiation detector 2 have the same angle ofincidence, which ideally is the Brewster angle of the liquid. In thiscase, the angle of incidence is defined by the angle between the lightbeam and the standard surface. The Brewster angle αis calculated usingthe formula tan α=n, where n is the refraction index of the liquid.Water, for example has a Brewster angle of 53°.

[0018] The intensity of the gloss passing through analyzer 9 isdetermined in the radiation detector 2. The gloss received from theradiation detector 2 is composed of the gloss of the surface of theliquid layer 12 and the gloss of the surface of the drum 10. By thesurface of the drum 10, for example, the curvature, texture and groovesof the drum 10 and deflected portions of the beam affect the glossreceived as well as diffused light, which in part comes from the coatingof the drum 10. The following differences exist between the gloss anddiffused light of the beam. Gloss maintains its direction; a parallelbeam hitting a surface generates a parallel beam leaving the surface;gloss remains polarized, since gloss is based on the specularreflection, and gloss has a certain preferred direction. Measures toincrease the gloss in the beam received are used, such as the use of aparallel beam from the radiation source 1, whereby the gloss remainsparallel, a sharp point arises in the focal plane. A position-locatingsensor in the radiation detector 2 locally receives a maximum; theillumination intensity at the radiation detector 2 is very high locally.If the radiation detector 2 is in the focal plane, then the image of alight spot is fuzzy and consequently attains a low illumination with theradiation detector 2. If polarized light from the radiation source 1 isused with a polarizer 8 and a polarizing analyzer 9 parallel to thepolarizer at the radiation detector 2, the gloss passes unimpededthrough the analyzer 9, while approximately half of the diffused lightis suppressed. If the beam strikes a very weak angle, the gloss portionis increased, but the diffused light portion remains the same.

[0019] Due to the specular reflection with gloss, the parallel course ofthe visible radiation is maintained. This leads to a sharp point in thefocal plane of the radiation detector 2. If a position-locatingradiation detector 2 is used, a narrowly defined maximum of theintensity is obtained. Contrary to gloss, diffused light is unpolarizedas a first approximation. Since the diffusion of the visible radiationusually contains no polarization, the diffused light has nopolarization. However, with many surfaces that the beam strikes, a majordirection of the diffused light can be recognized.

[0020] The radiation detector 2 is connected to a computer and a controldevice 3 and transfers the data concerning the intensity to them. In thecomputer and control device 3, the intensity of the beam received isdetermined, and the position allocation of the intensity is evaluated.Due to the determination of the local distribution of the intensity, thediffused light can be distinguished from the gloss, which is used todetermine the thickness of the oil layer or liquid layer 12.

[0021] In addition to the evaluation of the intensity of the beamreceived, an evaluation of the local spectral beam distribution of thegloss can be carried out in the radiation detector 2. In the case inwhich the light beam falls on surface of the drum 10 that has not beenwetted by liquid or oil, the diffused light is basically received by theradiation detector 2, and the surface of the drum 10 in the focal planeof the radiation detector 2 has a fuzzy image. In the firstapproximation, the diffused light is diffused in all directions. As afunction of the surface condition of the drum 10, there arises a certainpreferred divergent direction. Specifically, when a first measurement istaken, the surface without liquid is irradiated by the radiation source1; the reflected beam is received by the radiation detector 2, and dataconcerning the intensity of the gloss of the surface without liquid istransmitted to the computer and control device 3. Depending on thesurface to be measured, various intensity characteristics are produced.

[0022] Subsequently, the liquid is applied to the surface to bemeasured; during a second measurement of the surface with liquidirradiated by the radiation source 1, the beam reflected by theradiation detector 2 is received and data concerning the intensity ofthe gloss of the surface with fluid is transmitted to the computer andcontrol device 3. The data concerning the intensity of the gloss of thesurface with and without liquid are compared with one another in thecomputer and control device 3. The thickness of the liquid layer 12 isdetermined from the data concerning the intensity of the gloss in thecomputer and control device 3. To this end, for each gloss valueobtained by the radiation detector 2, a thickness of the liquid layer 12is allocated in an allocation table in the computer and control device3. The allocation table is compiled on the basis of the characteristiccurve of the gloss as a function of the thickness of the liquid layer12. The thickness of the liquid layer 12 can be reliably determined inthe above-described manner.

[0023] The dispensing device 7 which is connected with the computer andcontrol device 3, can be controlled by the thickness of the liquid layer12. If it is determined in the computer and control device 3 that theliquid layer 12 on the drum 10 is not thick enough for the chosenapplication, the dispensing device 7 can be controlled so that thedispensing of fluid is suitably increased.

[0024] Furthermore, the computer and control device 3 is connected to arotary encoder 15, which determines the rotation angle of the drum 10 atspecified distances, and transmits them to the computer and controldevice 3. In connection with the intensity of the beam received,measured with the same distances, the thickness of the liquid layer 12can be allocated to a specified rotation angle of the drum 10 andconsequently to the area corresponding to the rotation angle on thesurface of the drum 10, where the actual measurement of the degree ofgloss takes place. In this manner, the allocation of the thickness ofthe liquid layer 12 to the drum 10 can be measured by any rotation angleand the dispensing of the liquid, oil in this case, can be controlled bythe dispensing device 7, can be controlled by the computer and controldevice 3 depending on the rotation angle. A specific area on the surfaceof the drum 10, which has a low liquid layer thickness, can thus becontrolled by the computer and control device 3, which controls thedispensing of fluid on the metering roller 5, with the appropriateamount of fluid applied. In a similar way, the amount of fluid that isdispensed by the dispensing device 7 to the metering roller 5, can bereduced to the appropriate measurement, if the thickness of the liquidlayer 12 on a given area on the surface of the drum 10 is too great.

[0025] Furthermore, several radiation sources 1 and radiation detector 2can be arranged in axial direction to the drum 10, which measure thethickness of the liquid layer 12 independently from one another, inorder to determine the allocation of the thickness of the liquid layer12 along the rotating axis of the drum 10 and to control according tothe preceding description or the beam of the radiation source 1 can movearound axially along the drum 10 and the beam reflected by the surfacecan be deflected to the radiation detector 2 by means of a mirrorsystem. The last possibility has the advantage of only requiring aradiation source 1 and a radiation detector 2, and nonetheless, theentire axial length of the drum 10 can be measured.

[0026] When measuring the thickness of the liquid layer 12 on a surface,the diameter of the beam hitting the surface is normally not important.Since especially in the case of surfaces of printing drums or pressureplates, however, depressions or valleys with diameters in the range ofone to five micrometers exist, for the exact measurement of the liquidlayer 12 at all locations, including depressions and valleys, with aspecial embodiment, a light beam from the radiation source 1 isrequired, which has a corresponding small diameter in the range ofseveral micrometers, in particular, one or two micrometers, in order toalso detect the liquid layer thicknesses in the small depressions orvalleys in the surface.

[0027] With another embodiment, instead of the above-describedmeasurement of the degree of gloss, a measurement of diffused light iscarried out, which with optical systems, in themselves, is not desired.In this case, the radiation detector 2 receives reflected diffused lightfrom the surface by which the roughness of the surface is measured.Since the diffused light is basically reflected by the surface, thestructure of the surface without liquid can be determined with thediffused light. Another possibility for measuring the structure of thesurface without liquid is to use an optical profilometer. In particular,the structure of the surface of the printing drum is rough; when coatingwith a liquid layer 12, areas emerge that are covered with liquid andareas in which the surface of the printing drum that protrude from theliquid layer 12. The more the areas expand, in which the surface of theprinting drum protrudes from the liquid layer 12, i.e., the dryer thesurface is, the more the diffused light from the surface increases.

[0028] The above, described first measurement of the surface withoutliquid is thus carried out with a diffused light measurement, while thesecond measurement of the surface with liquid is carried out with ameasurement of the degree of gloss. With the diffused light measurement,the angle of incidence of the beam from the radiation source 1 to thesurface and the angle at which the beam is received from the surface tothe radiation detector 2, with each measured between the light beam andthe standard surface is changed in comparison with the measurement ofthe degree of gloss. The angle of incidence may, for example, amount to45° or 90°, the receiving angle 90° or 45°. In principle, the followingarrangements for the detection of the diffused light portion arereasonable. First, the radiation source 1 and the radiation detector 2have no polarizer 8 or analyzer 9. Second, the radiation source 1 has apolarizer 8 and the radiation detector 2 has an analyzer 9, whereby thespecular reflection is not suppressed, which means that the gloss withthe diffused light measurement from the radiation detector 2 isbasically not received. In the computer and control device 3, there is acharacteristic curve of the diffused light as a function of the surfaceroughness for the purpose of diffused light measurement. The evaluationof both measurements takes place as described above. The precedingdescription is only exemplary; the invention does not apply only to thedescribed measurements of an oil layer of an application roller of aprinter, but to the measurement of the thickness of a liquid layer 12 onany surface.

[0029] The invention has been described in detail with particularreference to certain preferred embodiments thereof, but it will beunderstood that variations and modifications can be effected within thespirit and scope of the invention.

What is claimed is:
 1. Method for measuring the thickness of a liquidlayer (12) on a surface, particularly for measuring the thickness of anoil layer on a printer, comprising: a radiation source (1) transmits abeam to the surface with the liquid layer (12), a radiation detector (2)receives beams reflected from the surface and the liquid layer (12), andthe thickness of the liquid layer (12) on the surface is determined bythe beam received on the basis of a measurement of the degree of gloss.2. Method according to claim 1, wherein the beam of the radiation source(1) is polarized perpendicular to the angle of incidence of the surface,and the polarization direction of the beam of the radiation source (1)and an analyzer (9) in front of the radiation detector (2) are the same.3. Method according to claim 1, wherein the rotation angle is measuredat specified distances and that the thickness of the liquid layer (12)is allocated to each rotation angle measured.
 4. Method according toclaim 1, wherein the angle of incidence of the beam from the radiationsource (1) corresponds to the Brewster angle of the liquid.
 5. Methodfor measuring the thickness of the liquid layer (12) on a surface,particularly for measuring the thickness of an oil layer on a printer,comprising: a radiation source (1) transmits a beam to the surface withthe liquid layer (12), a radiation detector (2) receives the beamreflected by the liquid layer (12), and the thickness of the liquidlayer (12) on the surface is determined on the basis of a diffused lightmeasurement by the beam received.
 6. Method according to claim 5,wherein the measuring of the thickness of the liquid layer (12) on thesurface is carried out with a diffused light measurement and ameasurement of the degree of gloss.
 7. Device for measuring thethickness of a liquid layer (12) on a surface, particularly formeasuring the thickness of an oil layer on a printer, comprising: aradiation source for transmitting a beam to a surface with the liquidlayer (12), a radiation detector (2) for receiving the beam reflected bythe surface and the liquid layer (12), and a device for determining thedegree of gloss and/or diffused light from the beam received.
 8. Devicefor measuring the thickness of the liquid layer (12) according to claim7, wherein the radiation source (1) and the radiation detector (2) arecarried out in one device.
 9. Device for measuring the thickness of theliquid layer (12) according to claim 7, wherein said printer has anapplication roller which is attached to a rotary encoder (15) formeasuring the rotation angle of the application roller.
 10. Device formeasuring the thickness of the liquid layer (12) according to claim 7,further including a screen on the radiation detector (2) to filter thediffused light.
 11. Device for measuring the thickness of the liquidlayer (12) according to the claim 10, further including an analyzer (9)in front of the radiation detector (2), which has the same polarizationdirection as that of the beam transmitted by the radiation source (1).12. Device for measuring the thickness of the liquid layer (12)according to claim 7, wherein the beam from the radiation source (1) hasa beam with a diameter in the range of a few micrometers, particularlyin the range of less than two micrometers.